Investigations of loaded crack tips in NiAl by atomic force microscopy
- Max-Planck-Inst. fuer Eisenforschung GmbH, Duesseldorf (Germany)
From the theoretical point of view, many crystalline solids have unexpectedly high fracture toughness values, but still fail by unstable cleavage. The loaded crack emits dislocations which shield the crack tip from the applied load but if the local stress exceeds the cleavage stress, the crack becomes unstable. This local stress depends on the dislocation distribution. Some dislocation nucleation and glide is thermally activated, this distribution, and therefore the brittle-to-ductile transition, depends on loading rate and temperature. The scanning probe microscopy techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have turned out to be powerful tools for atomic-scale surface characterization and should be potential tools for these measurements. However, to prevent oxidation, STM investigations must be performed in UHV, where positioning of the STM tip above the crack tip and loading of the specimen is difficult. Therefore the authors used an AFM to study the processes of crack tip blunting by dislocation emission and subsequent unstable crack propagation at laboratory air. A small bending device was constructed in which the specimens could be loaded in situ in the AFM. The measurements were done on NiAl single crystals which serve as a model substances for the mechanical behavior of intermetallic compounds. The crystals have B2 structure and, depending on orientation, a fracture toughness of approximately 3MPa{radical}m at room temperature. Only one type of slip vectors, <100>, operates in this alloy at room temperature and the preferred cleavage plane is {l_brace}110{r_brace}. With the AFM, the development of the elastic and plastic zone as a function of the applied load was examined in detail.
- OSTI ID:
- 237841
- Journal Information:
- Scripta Metallurgica et Materialia, Journal Name: Scripta Metallurgica et Materialia Journal Issue: 7 Vol. 33; ISSN SCRMEX; ISSN 0956-716X
- Country of Publication:
- United States
- Language:
- English
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